The invention relates to an electrical surface acoustic wave filter and, more particularly, to a filter having an improved out of passband suppression characteristic.
Surface acoustic wave (SAW) filters are known, for example, from the data book "Oberflaechenwellen-Filter LIOB" (Surface Wave Filter LIOB), issue 1983/84, published by Siemens AG of the Federal Republic of Germany. These SAW filters are integrated, passive components which have band filter characteristics that are function based on the interference of mechanical surface waves propagating along the surface of a piezoelectric material. The typical construction of SAW filters includes a moncrystalline, piezoelectric substrate, particularly an aluminum layer, such as by vapor deposition, piezoelectric input and output transducers, usually so-called interdigital transducers, formed into geometrical patterns by means of the photo etching technology. The interdigital transducers include several metallized, comblike electrodes, i.e. electrode with bus bars and fingers oriented perpendicular thereto and overlapping, in the active transducer areas, with respective interleaved fingers of the electrodes of different polarity. The substrate itself is glued to a metal carrier. Electrical connection between the input and output transducer and the terminals of the package for connection to external circuitry is typically accomplished via bond wires.
In operation, the surface acoustic wave filter converts an electrical signal applied to the input transducer into a mechanical surface wave which propogates on or in the substrate surface to the output transducer which then converts the surface wave back into an electrical signal with desired characteristics. The conventional transducers of the type referred to above have interdigital structures with transition time effects which could be provided with structures for shielding. Their properties are strongly frequency-dependent so that a filter effect for electrical signals is obtained due to the design of the structures.
The waves stimulated in the electrode overlapping areas of the interdigital transducers propagate not only in the desired propagation direction but, due to diffraction phenomena and due to the radiation distribution characteristic determined by the interdigital structures, also in the direction towards the edges of the substrate or chip. The waves traveling towards the short substrate edges are attenuated by conventional damping technology, using many different materials in particular photoresist and/or screen printing ink, applied to the respective substrate surface by the photoresist or screen printing method. The photoresist method has proven to be particularly well suited because the fine damping mass structures to be realized can be produced best rather conveniently with photolithographic techniques associated with photoresist.